Conventionally, as disclosed in Japanese Laid-Open Patent Application No.4-121260, a brake force control apparatus is known which increases a power ratio of a brake booster when a brake pedal is depressed at a speed exceeding a predetermined speed.
A driver of a vehicle operates a brake pedal at a high speed when a rapid increase in a brake force is desired (hereinafter, this operation is referred to as an emergency brake operation). According to the above-mentioned brake force control apparatus, it is determined that the emergency brake operation is performed if an operating speed of the brake pedal is larger than a predetermined value, and a brake force larger than that of a normal time is obtained by increasing a pressure of brake fluid stored in a reservoir and supplying the brake fluid to wheel cylinders. Hereinafter, this control is referred to as a brake assist control (abbreviated as BA control).
According to the above-mentioned brake force control apparatus, a power ratio larger than that of a normal time can be generated when an emergency brake operation is performed so that an advantageous state is established in rapidly increasing a brake force. Accordingly, when the driver performs an emergency brake operation, a brake force which appropriately responds to a request of the driver can be generated.
In order to generate a brake force larger than that of a normal time when the driver performs an emergency brake operation, means for increasing brake fluid must be provided to the brake force control apparatus. Various means can be considered as such means for increasing the pressure of the brake fluid. A pump is considered to be one of the various means. It is appropriate to change a tendency of rising of a brake force.
The conventional brake force control apparatus generates a brake force larger than that applied in a normal condition by always changing the power ratio of the brake booster in the same manner when an emergency brake operation is performed. Thus, when an emergency brake operation is performed, a brake force to which the driver's intention is reflected cannot be generated by the above-mentioned conventional brake force control apparatus.